The present invention relates to the field of manufacture of optical data storage disks. More particularly, the present invention relates to a method and apparatus for assembling a hub to an optical disk with minimal mechanical runout on a mass production basis.
Optical data disks are a popular media choice for the distribution, storage and accessing of large volumes of data. Examples of information stored on optical data disks include audio and video program material, as well as computer programs and data. Formats of optical data disks include audio CD (compact disk), CD-R (CD-readable), CD-ROM (CD-read only memory), DVD (digital versatile disk or digital video disk) media, DVD-RAM (random access memory), various types of rewritable media, such as magneto-optical (MO) disks (including near field recording technology) and phase change optical disks.
A typical optical disk assembly includes a plastic optical disk substrate and a hub. The optical disk substrate includes opposing, first and second major surfaces and a central opening. For most applications, data tracks are formed in the first major surface (or reference surface), although data tracks may also be formed in the second major surface. The hub is adhered to the second major surface about the central opening with an adhesive such as a glue. During use, a chuck associated with a disk drive engages the hub and rotates the optical disk assembly by rotatably driving the hub.
Various techniques and processing equipment have been employed to assemble the hub to the optical disk substrate. As a general statement, these techniques are based upon prior art methodologies for mounting a hub to metal-based disk substrate (e.g., an aluminum-based hard disk). Thus, for example, a hub is typically affixed to the surface of an optical disk with an adhesive (e.g., a glue bead). The mounting location for the hub on the optical disk is typically near or at the inner diameter of the optical disk surface. For example, the disk substrate may be generally supported at the bottom (or first major) surface while the hub is axially aligned with the central opening and pressed into contact with the adhesive and the top (or second major) surface of the disk substrate. The fixture used to support the disk substrate is independent of the assembly used to press the hub into engagement with the adhesive/disk substrate. Further, during the fitting process, only a top portion of the hub is supported.
It is vital that the hubbing process result in the hub being planar with the reference (or first major) surface of the optical disk substrate. To this end, advancements in optical disk technology and the demand for increased disk capacity has resulted in a greater amount of information being stored within the same sized area of a disk surface. Such high capacity optical disks require more complex optical disk readers/recorders. Near field recording is one form of optical recording that is capable of producing extremely small spot sizes, for example, on magneto-optic disk media. For near field recording, a solid immersion lens (SIL) can be used to transmit an optical beam across an extremely thin air bearing, and through the top of the recording medium onto the recording layer. The beam is "air-incident" in the sense that it does not pass through the disk substrate before it reaches the recording layer. This aspect of near field recording differs from the substrate-incident techniques used in conventional magneto-optic recording, in which the beam passes through the substrate. A SIL can be integrated within a flying magnetic head assembly that hovers above the optical disk during operation and provides the magnetic bias for magneto-optic recording. For near field recording, the thickness of the air gap is less than one wavelength of the recording beam. Because of the tight physical characteristics of the near field recording process, it is critical that the hub be mounted parallel to the surface of the optical disk substrate. Since the gap between the flying magnetic head assembly and the surface of the disk is less than one wavelength of the recording beam (two to four microinches), excessive or any tilt in the optical disk during operation can result in a head crash (i.e., physical contact of the head with the disk) or disk drive failure.
Although the importance of hub-to-disk surface planarity has been recognized, currently available hubbing techniques have not kept pace with the advancements in optical disk media described above. That is to say, for end use applications that can tolerate minor deviations in hub planarity, the press fitting-type hub assembly processes are acceptable. However, where even a slight deviation in hub/disk surface planarity cannot be tolerated (such as described above), fitting a hub onto the disk surface without evaluating or otherwise accounting for inconsistencies in disk substrate planarity will likely result in an unacceptable product.
The main issue underlying unacceptable hub assembly is the fact that for a plastic-based disk substrate, the opposing major surfaces are virtually never planar. Unlike a metal-based substrate, the opposing major surfaces of an optical disk substrate are never precisely planar at any one location, even though the disk substrate is molded and grounded to strict tolerances. As a result, the opposing surfaces are essentially not parallel. Standard hubbing techniques assume, however, that the opposing surfaces are planar and parallel. For example, a typical hubbing device includes a flat disk support surface and a hub placement device. The disk substrate is laid on top of the flat disk support surface such that the first major surface (or reference surface) is supported. The hub placement device orientates an individual hub such that the hub is planar relative to the flat disk support surface and guides the hub into contact with the second major surface of the disk based upon this assumption. However, as described above, the first major surface of a plastic disk substrate will not be precisely parallel or planar relative to the flat disk support surface. Additionally, the second major surface of the disk substrate will also not be perfectly parallel or planar relative to the flat disk support surface or the first major surface. As a result, orientation of the hub relative to the flat disk support surface effectively bears no precise relation to the actual planarity of the second major surface. Further, by undiscernibly fitting the hub to the second major surface, orientation of the hub is entirely unrelated to planarity of the first major surface, into which data tracks are formed. Thus, it is virtually impossible for the hub to be precisely planar with the first major surface as the hub is essentially permanently orientated to be planar with the flat disk support surface and/or the second major surface, leading to axial and radial run-out problems. Additional performance concerns may arise due to the deflectability of the plastic-based disk substrate (e.g. dishing) and concentricity of the hub relative to the disk substrate.
Optical data disks continue to be extremely popular for storing large volumes of data. To this end, a plastic-based optical disk offers high performance capabilities at a relatively low cost. However, inherent imperfections in the disk substrate itself greatly hinder hub assembly within certain tolerance ranges using available mounting techniques. Therefore, a substantial need exists for a mass production method and apparatus for assembling a hub to an optical disk substrate that produces optical disk assemblies with minimal axial and radial runout, optimal concentricity and limited dishing.